1. Field of the Invention
This invention relates generally to fluid pressure relief valves and, more particularly, to a relief valve wherein the pressure drop across the valve is substantially constant and the fluid circuit with which the valve is associated drains through the valve when the circuit is unpressurized.
2. Description of the Prior Art
Relief valves are used in fluid pressure circuits to establish maximum circuit pressure. In typical simple relief valves where a valve element is spring biased to a closed position and forced open by fluid pressure above a maximum value, flow through the valve varies directly with the pressure gradient across the valve. In order to increase the rate at which fluid exhausts, the pressure gradient across the valve must increase. In addition, where the fluid circuit is a transmission hydraulic control circuit including a filter chamber which does not drain when the circuit is unpressurized, the task of changing the filter is rendered more difficult by the residual fluid in the chamber. A relief valve according to this invention, once open, responds to increased flow rate to decrease the resistance to additional valve element movement so that the pressure gradient across the valve is substantially constant and, in the closed position, provides a convenient circuit drain when the circuit is unpressurized.